1. Field of the Invention
The present invention relates generally to compositions and methods for controlling precipitation when treating oil and gas wells with an acid solution, and more particularly, but not by way of limitation, to compositions and methods for controlling precipitation of ferrous sulfide and free sulfur when treating sour oil and gas wells with an acid solution.
2. Description of the Prior Art
Acid solutions are used to treat oil and gas wells in many ways. For example, acid solutions are commonly injected through the wellbore into the formation to increase the permeability of the formation and to stimulate production of oil and gas therefrom. Acid solutions are also used in cleanup processes carried out in connection with various types of wells and associated piping and equipment.
Iron precipitation has been a problem encountered in acid treatment operations for many years. The acid dissolves iron containing scale from pipe and equipment and iron containing minerals present in the formation. As the acid spends and the pH of the acid solution increases, iron present in the solution in the ferric, Fe (III), oxidation state precipitates as ferric hydroxide (Fe(OH).sub.3). The ferric hydroxide precipitate can plug the formation and cause other serious damage.
Precipitation of ferric hydroxide can be controlled by adding an iron sequestering and/or iron reducing agent to the acid. Iron sequestering agents such as citric acid and ethylenediaminetetraacetic acid (EDTA) chelate the iron and hold it in the solution. Iron reducing agents convert ferric iron present in the solution to the less-damaging ferrous state.
When a well being treated with an acid solution is sour, controlling precipitation of ferric hydroxide is not enough. A well is sour when it contains a significant amount of hydrogen sulfide. The combination of sulfide ions from hydrogen sulfide and iron creates additional precipitation problems.
Sulfide ions reduce ferric ions to ferrous ions by the following reation: EQU b 2Fe.sup.+++ +S--.fwdarw.S.degree..dwnarw.+2Fe.sup.++
This reaction is beneficial in that ferric ions are reduced to ferrous ions and less precipitation of ferric hydroxide occurs. Yet, the reaction is detrimental in that elemental or free sulfur is formed. Free sulfur can plug the formation and cause other serious damage.
In addition, sulfide ions react with ferrous ions to form ferrous sulfide, FeS. Ferrous sulfide scale deposited on tubing and casing is often dissolved by the acid and carried into the formation. Also, sulfides present in the formation react with ferrous iron in the acid solution. When the acid reacts with the formation and spends to a pH of about 1.9, a pH well below the pH of completely spent acid, the ferrous sulfide in the acid solution precipitates. Like free sulfur, ferrous sulfide precipitate can plug the formation and cause other serious damage.
Thus, precipitation problems encountered in acid treatment operations carried out on sour wells are more complex. When treating a sour well with acid contaminated with iron, precipitation of free sulfur and ferrous sulfide as well as precipitation of ferric hydroxide must be controlled.
Previous attempts to control precipitation of free sulfur and ferrous sulfide when treating a sour well have been only partially successful.
Attempts to control precipitation of ferrous sulfide by maintaining the pH of the acid solution below 1.9have failed. In most acid treatment operations, it is not feasible to maintain the pH of the acid solution below 1.9 during the entire operation.
In U.S. Pat. No. 4,633,949, a method and composition for preventing precipitation of ferrous sulfide and sulfur when acidizing a sour well are disclosed. An iron complexing agent such as ethylenediaminetetraacetic acid (EDTA) and an iron reducing agent such as erythorbic acid are added to the primary acidizing solution. The iron complexing agent is added to stabilize the ferrous ion in solution in order to inhibit precipitation of ferrous sulfide, FeS. The iron reducing agent is added to reduce ferric ion to ferrous ion in order to inhibit formation of elemental sulfur. Unfortunately, the reducing agent does not adequately prevent the formation of elemental sulfur. Precipitation of both elemental sulfur and ferrous sulfide is not adequately controlled by merely controlling the iron present in the acid solution.